In order to construct an interactive application using computer graphics, it is necessary to display image information in real time on a display device to present the image information to the user. When rendering a three-dimensional virtual environment which is viewed from a virtual viewpoint as an image presented to the user, the computer needs to carry out a complex computation, such as a projection process, by using many pieces of information including the geometric shape and the colors of the three-dimensional virtual environment, etc.
However, such a three-dimensional virtual environment as one expressing a wide urban area includes a large number of buildings having a simple shape, such as right prisms, and has many models. Therefore, such a three-dimensional virtual environment includes many polygons and the amount of computation at the time of rendering increases. Further, when individually assigning an image called a texture to the surface of each building, it is necessary to alternately send a command for switching between textures and a command for rendering a polygon to graphics hardware. On the other hand, because typical graphics hardware is optimized in such a way as to carry out many rendering processes in response to one command, the rendering speed decreases when receiving many commands.
A three-dimensional image display device disclosed by the following nonpatent reference 1 carries out such a process as shown below in order to render a lot of building models at a high speed and with a high degree of quality. When receiving a set of building models each expressed by a polygon model having a right prism shape (referred to as a “right prism model” from here on) first, the three-dimensional image display device selects a group of right prism models which is a target for approximation from the right prism models expressing the building models, and unifies the group of right prism models to generate an approximate right prism model recursively. The three-dimensional image display device thus renders an approximate right prism model instead of a plurality of right prism models existing at long distances from the point of view, thereby reducing the number of polygons rendered and the number of times that the three-dimensional image display device switches between textures, and implementing high-speed rendering. Further, because right prism models that are replaced by an approximate right prism model at the time of rendering are located at long distances from the point of view, the degradation of the rendering result can be reduced.
However, according to this method, each approximate right prism model has five textures for roofs, and either one of these different textures for roof is used according to a positional relationship with the point of view at the time of rendering. As a result, the amount of texture data increases. Further, because the three-dimensional image display device recursively unifies right prism models on a two-by-two basis, the number of approximate right prism models generated increases and hence the data volume of groups of approximate right prism models generated reaches several times as large as the inputted right prism model group.
On the other hand, because the volume of a storage unit from which data can be read at a high speed, such as a memory or a VRAM, is relatively small in the structure of a typical computer, it is difficult to hold all of a large volume of data at the time of rendering. Particularly in a terminal having a relatively low performance, such as an embedded device, because the volume of a high-speed storage unit installed therein is further small, it is difficult to use the above-mentioned method.